DESCRIPTION (adapted from the application) Healthy cells receive and respond to information from their external environment. Failure to connect external information with internal behavior is one of the hallmarks of malignancy. For example, epithelial cells in the colon use spatial cues from neighboring cells to produce uniform polarity and directed migration, but during tumorigenesis they lose this organization. One way for cells to integrate spatial signals is to link the submembrane cortex with the internal cytoskeleton. An interesting set of proteins potentially involved in providing such a linkage are those that specifically localize to the distal tips of microtubules. Several microtubule tip binding proteins have recently been identified. One of these is EB1, the protein on which the K08 proposal referenced above is based. Others include the endocytic vesicle linker protein CLEP 170, the kinesin-like MCAK/XKCM1, APC, and components of the dynactin complex. I have shown that in yeast, the EB1 homologue, Bim 1p resides at the tips of microtubules and regulates orientation of the mitotic spindle. Bim lp's role in spindle orientation appears to be twofold: it regulates microtubule dynamics and it interacts physically with the cortical protein Kar9p. In this proposal, I outline a series of experiments in Xenopus egg extracts and tissue culture cells that focus on the interactions of EB1 at the microtubule tip. First, I will determine biochemical relationships between EB1 and known as well as previously uncharacterized microtubule end binding proteins. Second, I will determine the functional roles of these proteins in orienting the mitotic spindle in polarized epithelial cells, where they are likely to coordinate spindle orientation with external polarity cues. I also explore new models of microtubule-cortex interactions. Insights gained from these experiments will facilitate rational drug design for anti [sic] cancer therapies and a greater understanding of cancer and developmental biology.